Pressure-sensitive adhesive compositions find use in a wide variety of applications, such as the automotive, aerospace, construction and electrical markets, either in the form of tapes or as adhesive coatings on other backings. For many of these applications, good flame retardant properties are of considerable importance.
U.S. Pat. No. 3,515,578 (Tomita) describes an acrylate-based pressure-sensitive adhesive composition comprising tris-(halogenated alkyl) phosphate and antimony oxide. U.S. Pat. No. 4,061,826 (Petras) describes a pressure-sensitive adhesive composition for use in an electrical insulating tape where the adhesive comprises highly halogenated organic materials such as decabromodiphenyloxide. WO 90/01411 (La Mar) describes a double-coated cloth adhesive tape for the aircraft industry having a flame retardant system in the pressure sensitive adhesive. The single flame retardant agent described is FYROL CEF available from Stauffer Chemical. This is a chlorinated phosphate (tri (beta-chloroethyl) phosphate)).
Halogenated flame-retardant agents, however, have considerable drawbacks in terms of smoke production and toxicity concerns, relating both to combustion products and also to disposal of the compounds themselves. A further limitation of the use of halogenated flame retardant agents in pressure-sensitive adhesives is that during combustion they emit larger amounts of corrosive smoke. Combustion products of halogen containing materials, in particular HCl and HBr, contribute significantly to these effects. The corrosivity of smoke is of particular importance in electrical/electronic applications, since a small fire involving halogenated insulation materials may generate sufficient corrosive combustion products to cause widespread dame to other equipment not directly involved in the fire.
In case of pressure-sensitive adhesive tapes supported by a backing, it has also been suggested to incorporate the flame-retardant system into a polymeric film backing of the adhesive tape rather than into the pressure-sensitive adhesive itself. JP 05-345,882 describes a flame agent retardant pressure-sensitive adhesive tape containing a flame retardant system selected from the group of 1) hydrates of metal oxides such as magnesium hydroxide and alumina trihydrate and 2) red phosphorus. The flame retardant system is incorporated into the polyolefin-based backing of the adhesive tape.
This approach is, however, not applicable to unsupported layers of adhesive (transfer tapes) which are becoming increasingly useful as components for mounting and attachment of surfaces. In this situation, if flame retardance is desired, it must be introduced into the pressure-sensitive adhesive itself as there is no backing or supporting film present.
Consequently, there is a need for providing flame-retardant pressure-sensitive compositions with an acceptable or good overall balance of properties using non-halogenated flame-retardant systems or agents.
Flame-retardant systems can be classified according to whether they exhibit intumescent or non-intumescent properties. Pressure-sensitive adhesives containing an intumescent, fire-retardant system, create when burned, a foamed char layer, which impedes heat-transfer and may operate as a sealant thus suppressing spreading of fire. Intumescent, fire-retardant systems are typically complicated formulations usually comprising a "carbonific" part, i.e. a source of carbon, an acid source and a "spumific" part (or blowing agent) evolving gaseous products which blow the char up into the form of a foam. These functions are usually provided by a system comprising three or more compounds. An exception is ammonium polyphosphate (APP) which according to J. Troitzsch, Makromol. Chem., Makromol. Symp., 74 (1993), 125-135, p. 131, section `Intumescent Flame-Retardant Systems` provides both acid and spumific functions. An intumescent system comprising APP can therefore comprise two or more compounds.
Pressure-sensitive adhesives containing a non-halogen intumescent flame-retardant system are described in WO 94,11,425 filed on May 19, 1995, by the present applicant. This reference lists on page 4 a number of commercially available intumescent formulations and cites further formulations described in literature.
There are, however, many applications such as, for example, bonding of non-functional surfaces in the transportation or building and construction industry where flame retardance properties are essential while intumescence is not. Since intumescent flame-retardant systems are typically relatively complicated formulations, it was desirable to provide non-intumescent pressure-sensitive adhesive compositions comprising non-intumescent, flame-retardant systems comprising one or more flame-retardant agents. It was also desirable to provide another class of flame-retardant pressure-sensitive adhesive compositions as an alternative to the compositions described in WO 94/11,425 to enlarge the pool of flame-retardant pressure-sensitive adhesive compositions which is accessible to the person skilled in the art.